This invention relates to a method and apparatus for tumble-coating particles, and in particular, to a significantly improved method and apparatus which is especially suitable for the continuous coating (in a drum-like tumbler-blender) of wood particles with one or more sprayed coating agencies, such as a liquid (or powder-form) resinous binder, and/or a liquid wax. For the purpose of illustration, a preferred embodiment (and several modifications) of the invention, and a preferred manner of practicing the same, are described herein particularly in the context of coating wood particles with a resinous liquid binderxe2x80x94specifically, coating such particles in preparation for sending the blended/coated material into a former for making composition board.
In the usual manufacture of composition board of the type formed from a blend of wood flakes, slivers, fines and other wood particles, these wood-constituent components (furnish), prior to their being formed into a finished manufactured, composition board product, are conventionally coated with one or more coating materials, such as (1) a suitable resinous liquid binder, (2) a wax, and/or (3) possibly various other related or different additive substances (liquid or powder). In relation to the use and handling of liquid resin and wood furnish during such a coating operation, several significant coating-efficiency problems existxe2x80x94problems which reveal themselves, among other ways, in undesirable clumping in the flow of coated product, and, effectively, in wasted use of resin and wood furnish.
In particular, liquid binder resin, a noticeably expensive material, is usually oversupplied in today""s particle-coating equipment as part of an attempt to correct observed inefficiencies in coating and in material flow-management, and to assure proper, thorough, final particle coating. Oversupplied resin however introduces significant wastexe2x80x94a waste which adds to the cost of making composition board both in relation to the expense of the binder material per se, and in the issues which arise from the problematic accumulation and accretion deposit of cured and curing resin on various surfaces in tumble-coating machinery. Such accumulation necessarily requires frequent, recurrent removal of the unwelcome deposit material, and the task of removal is very time-consuming and very expensive.
A large troublesome issue (already mentioned) which is associated with conventional practice is the fact that a conventional output flow of coated wood particles is often characterized by clumping unevenness (so-called doughnutting)xe2x80x94unevenness which evidences itself as irregular amassing and clumping of output coated product. Such clumping comes about at least in part as a consequence of the mentioned resin oversupply which usually takes place, and further, as a consequence of the halting, rather than the smoothly forward-flowing, transport of coated particles in a drum. Material clumps in such an output flow typically each take the form including a central core/mass of poorly, or even non-coated furnish particles, surrounded by a jacketing mass of heavily overcoated furnish particles.
Despite various prior art efforts to eliminate or minimize this clumping/doughnutting difficulty, the problem remains as one that requires creative attention and resolution.
Given this situation, the present invention (as illustrated and described herein) focuses attention on improving the efficiency of drum-like tumbler-blender apparatus, and also of the method involved in the environment of that apparatus, wherein coating of wood-constituent components, prior to their being assembled into composition board, takes place. In particular, it focuses attention: (1) on offering improvements which result in significantly better (more thorough and complete) wood-particle coating; (2) on eliminating the above-mentioned clumping problem chiefly through promoting significantly evenized material flow through a tumbler-blender; (3) on coffecting the wasteful use of resin (and any other additive coating materials); and (4) on doing all of these things in an inexpensive and practical manner.
According to the present invention, these important issues and concerns are addressed by introducing, into an otherwise conventional tumble-coating environment, one or more unique baffle structures that include baffle surfaces which effectively change the tumble-flow paths of particles circulating in a rotary drum in ways which maximize the efficiency of the coating that can be achieved in the drum. While various prior art baffling schemes have been proposed in the past, none offers the advantages that are confirmedly gained by employing the baffling organization specifically proposed by the present invention. The baffling arrangement and method employed by this invention creates a particle-flow condition in a rotary, tumbler-blender structure, such as a drum, which promotes even, non-clumping material flow in that structure, and which further tends to cause substantially all resin which is introduced, for example, by way of a conventional spinning spray structure, to be fully engaged by particles and thus to be substantially fully used and not wasted as a troublesome drum residue which later must be dealt with. Excess resin does not need to be supplied.
According to a preferred embodiment of the invention which utilizes a rotary drum with plural, longitudinally distributed particle-coating zones, each such zone is occupied by a unique baffle structure including one or more specially designed and positioned baffle surfaces that modify the otherwise nominal generally circular tumbling flow of particles in that zone. Most importantly, the invented baffle structure, by altering tumble-flow activities in certain ways that involve the promotion of substantially constant and uniform flow and advancement of material through the drum, dramatically evenizes the output material flow from the drum. It substantially eliminates clumping/doughnutting as a direct consequence of such improved, regularized material flow.
In the simplest form of this preferred embodiment of the invention, a single baffle surface only is employed which creates a generally linear curtain flow of particles on that side of each coating zone which is characterized by downward motion of the nearby wall of the rotating drum. This baffle surface, which preferably is positionally adjustable, redirects particles that enter an upper quadrant in a drum by engaging them and coaxing them toward the opposite side of the coating zone, near the base of which is located a conventional spinner head which distributes coating material, such as a liquid resin.
A more elaborate form of the invention includes, in addition to a baffle surface (or surfaces) of the kind just mentioned, another baffle surface which takes generally the form of a downwardly flaring, truncated cone that acts like a shroud overhead the spinning coating-spray-dispensing nozzle (or head). Each such shroud creates a cylindrical, gravitational shadow region which remains substantially free of particles and a moderately broad (diametrically), cylindrical curtain-fall of particles spread apart in an unconventional manner relative to the associated resin-spray spinner head. Such spreading of particles promotes the invention""s ability to create more evenly and thoroughly coated wood particles than are attainable in conventional tumble-coating apparatus.
Yet another important form of the invention includes still another kind of baffle organization which further includes an elongate, inverted V or Y, tent-like arrangement that sits overhead the typical long feeder manifold conduit which carries liquid spray material into each coating zone. This tent-like structure further helps to change the nominal tumble path of particles by engaging certain particles and deflecting them into laterally-spaced curtain flows. Such tent-like structure, in addition, prevents another conventional and troublesome phenomenon known in the art as xe2x80x9cbird nestingxe2x80x9dxe2x80x94a tendency of coated particles to build sticky clumps on top of the manifold conduit just mentioned.
In the drawings herein which illustrate the most preferred form of the invention, that form includes all three types of these just-mentioned baffle-structure components.
The various features and advantages of the present invention, some of which have just been discussed briefly, will become more fully apparent as the description of the invention which now follows is read in conjunction with the accompanying drawings.